Stuffing a big engine in a small car may not be a new trick, but whenever a car manufacturer tries the old presto-change-o with a factory engine swap, enthusiasts will be in the first row to see if the magic works. Audi donned the top hat and cape to wave its magic wand over the A4, filling the engine compartment with the 2.8-liter V6 used in the larger A6; Audi reduced the size of the cylinder bores, then strapped on a pair of diminutive turbochargers and matching intercoolers. With some hocuspocus for the suspension, an abracadabra over styling, a flash of mirrors in the interior and a cloud of smoke around its big brakes, Audi has pulled not a rabbit but a lion out of its A4 hat. It named this new lion the S4.

Audi S4 comes as one $37,900 model available with a choice of transmissions: 6-speed manual or, as a no-cost option, 5-speed Tiptronic automatic. All S4s come with Audi's quattro all-wheel-drive system, a 250-horsepower twin-turbo V6, massive 225/45R17 tires and leather upholstery.

The S4 is, at its core, a modified A4. Thus it retains the four-door configuration and general dimensions of the smallest sedan Audi offers in the U.S. Differences begin under the hood. Like all Audi engines sold in the U.S., the S4's cylinder heads have five-valve-per-cylinder architecture for superior breathing. Two small turbochargers nestle out of sight under each bank of the vee on either side. Two small turbos can deliver the same boost as one larger turbine because, with less mass, they spool up faster. It virtually eliminates annoying the lag, or delay in throttle response, associated with big turbos.

Audi did more than hang twin turbos on its V6, however. The engineers also made the cylinder bore slightly smaller -- and thicker -- to handle the added pressure from turbocharging; that reduced displacement from 2.8 to 2.7 liters. Carefully shaped intake tracts cause the intake charge to tumble in the combustion chamber for more efficient burning, and the intake valve timing is variable. The engine is tuned for responsive torque and develops 258 foot-pounds from 1850 to 3600 rpm; it reaches its horsepower peak at a relatively low 5800 rpm.

Audi's quattro system features a TORSEN differential that mechanically redistributes torque up to 66 percent to whichever axle has more traction. At both front and rear the S4 has Electronic Differential Locking, which detects and limits wheel spin from side to side up to 45 mph. Audi brags that with the current quattro system, its cars can get underway with traction at only one wheel.

Audi revised the suspension of the S4 with extensive use of aluminum. Through the magic of geometry, the patented four-link front suspension sharpens steering response. All the control arms of the front suspension, the ball joints, front suspension wheel carriers and double wishbone rear suspension are made of forged aluminum. The lighter suspension pieces mean a smoother ride and improved tracking of wheels over bumpy pavement for better grip. The sport suspension tuning lowers the S4 and includes firmer springs and shock absorbers. The S4 sports new brakes, with massive 16-inch discs with 4-piston 4-pad calipers inside 17-inch Avus alloy wheels mounted with 225/45 performance radials.

The S4 doesn't shout its presence (unless one chooses the Imola Yellow or Nogaro Blue pearl effect paint). Visual differences from the A4 models are limited to the wheels and tires, larger air intakes at the front of the car, standard xenon headlamps with clear glass covers and subtle badging.

The S4 interior shares its design with the rest of the A4 line, which is to say the days of dark and foreboding German interiors is over. Buyers can choose from a variety of interior treatments called "Atmospheres." Our test S4 had carbon fiber-look accents, which look right at home in this high-performance sedan. Special to the S4 are sport seats with deep side bolsters designed to keep driver and passenger in place during vigorous driving. The seats are multiadjustable and just about everyone should be able to get comfortable. The seats are covered with pearl nappa leather, along with the rear seat, armrest, and door panel inserts. It's available in either a dark onyx or silver or, for no cost, with Alcantara suede inserts in either silver or a striking blue.

Audi paid particular attention to the driver's office, with large round easy-to-read instruments under a blister on the dash. The steering wheel is contoured with thumb grips at the proper nine and three o'clock positions, but the surface is harder and slipperier than we'd like.

The back seat is comfortable, but this is a compact car so rear legroom is more limited than in bigger sedans. Although there are three seatbelts in back, three adults will find shoulder room inadequate. This is a four-grownup car.

Regardless of the ingredients, the proof is in the pudding or, in this case, the magic brew. And magic it is. The S4 idles quietly and smoothly, belying the power that won't be revealed in casual around-town driving either. But yield to the temptation to lay into the throttle and the Audi surges ahead, even with revs as low as 2000 rpm. There's a solid push throughout the rev range, with neither turbo lag nor turbo rush. There's no turbo whistle either, just a muted throaty roar from the V6. Motorheads-who are most likely to buy this car-might actually wish the engine was a bit louder under full throttle. The twin-turbo makes quick work out of passing on a two-lane road and it catapults from corners. Without attention to the speedometer, one could easily become radar fodder. In this case, you'll appreciate that conservative exterior.

Although the S4 is available with a 5-speed Tiptronic automatic, we drove only the 6-speed manual transmission. Some 6-speed gearboxes are difficult to use, but there was no such problem with the S4. The only possible complaint is that the fore-and-aft shift throws are somewhat long. Clutch engagement and disengagement are abrupt, not surprising given the amount of power available. Practice will be required for smooth starts and shifts.

Suspension is pleasantly firm. The S4 has the kind of ride that traditional luxury car buyers would call hard. Indeed, compared to even, say, an A6 2.7 T, with which the S4 shares its engine, the S4 rides much more firmly. Road seams, blemishes and flaws that the A6 would absorb, the S4 telegraphs to the occupants. On an undulating road the S4 feels like a speedboat on a light chop. The low-profile 17-inch tires relay a lot of road noise from coarse pavement, seams and bots dots. The payback comes in the crispness with which the S4 attacks corners. The steering has a nice Germanic weight to its feel, a one-to-one correspondence with the road and very precise. The S4 rotates well; even under power coming out of corners. Where other cars would drift wide through a corner, the S4 will continue to turn. Enthusiasts will want to arrange some track time.

A racetrack would be necessary to fully test the massive brakes on the S4. Earlier Audi performance models were shipped without the Autobahn-ready binders because they thought Americans didn't need them. We were unable to elicit any fade from the four big stoppers even when making repeated stops from high speed. If anything, they improved with a little heat in them. We suspect that even track testing would find these brakes unflappable.

This car is clearly intended for driving enthusiasts. The extra expense of an S4 over an A4 2.8 would not return equal extra enjoyment to someone not interested in exploring the S4's extra capabilities. In fact, the harder ride would probably make the S4 less rewarding. Not that the S4 wouldn't be perfectly content to trundle about in everyday traffic. It just seems a waste of good magic on mundane tasks.

But for an enthusiast, Audi's S4 is an outstanding ride.